When my Odroid C2s started crunching 2^64 WUs they used 800MB to 900MB RAM, those that finish now use 1,500MB to 1,600MB.

However, WUs are now frequently either failing to finish (today 10 reported back to the project that there is not enough memory on the client), or are locking up the computer completely (SSH and BOINC Manager remote access attempts both time out) and they have to be power-cycled to regain access, or the WU does not run and is shown as "waiting for memory".

This behaviour started a few days ago, and has steadily increased in frequency to the point I've had to stop trying to run 2^64 WUs.

Are machines with 2GB RAM no longer suitable for this project?

Yes, it looks like the 2^64 app requires more memory per thread now.